1. Field of invention
The process entails feeding an aqueous urea solution to or creating and aqueous urea solution in a hydrolyser (reactor) where it is heated under pressure to produce ammonia gas mixture of ammonia, water and carbon dioxide. In this invention the reactor operating pressures and temperatures are allowed to vary to match ammonia demand requirements. The reactor pressure is set either to vary with ammonia demand or with reactor operating temperature. The operating pressure is varied such that during normal operation the concentration of these gases remains in equilibrium with the composition of the solution in the reactor and equal to the conversion products of the feed composition. In a typical process the feed solution is in range of 40 to 50 weight percent urea to water.
2. Brief Description of Related Technology
Utilities and A/E firms have shown keen interest in technology for urea-to-ammonia conversion. Utilities are increasingly adopting urea as a preferred alternative to anhydrous and aqueous ammonia for their SCR projects, with several major utilities committing to the urea-to-ammonia alternative and many potential users actively evaluating systems for current and future projects. Other processes such as SO2 scrubbing with ammonia are evaluating the use of urea to ammonia. The improvements of this invention make this more feasible by providing the means to significantly reduce the heat requirements for the production of ammonia from urea. Other applications are also evaluating the use of ammonia production to reduce the onsite storage of ammonia.
The development of urea-to-ammonia technologies is in response to the greatly increased requirements for utilities to control NOx emissions and their implementation of SCR projects that require ammonia as the reducing agent. Anhydrous ammonia is regarded as a hazardous and toxic chemical and is subject to stringent regulations imposed by the EPA as well as OSHA. Aqueous ammonia, although less concentrated, poses similar risks, and is also becoming increasingly regulated or subject to restrictions by local authorities. The use of aqueous ammonia as an alternative to anhydrous, significantly increases operating costs for chemical and energy, and increases transport and storage requirements. These disadvantages are exacerbated as more dilute aqueous solutions are considered.
The urea to ammonia and other on demand urea to ammonia system uses urea as the feedstock chemical and thereby entirely avoids risks associated with the transportation and storage of ammonia. The process transforms urea solution to an ammonia gas mixture on demand to meet the dynamic requirements of the NOx control system and other systems using ammonia.
The initial systems were designed for operating temperatures of 300 deg F. and operating pressure of 60 to 80 psig with a urea feed concentration of 40%. Higher urea feed concentrations reduce the operating cost by decreasing the energy required to evaporate the water in the feed solution. As the market matures higher temperature designs and higher 50% urea feed concentrations can reduce the capital cost of the system as well as reduce energy consumption.
Since the rate of hydrolysis of urea is increased with excess water, maintaining adequate water in the reactor liquid is essential to the process. However, for 50% urea feed and above, the potential for ammonia production rate to be slowed because of diminishing water for the reaction must be considered. The Spencer, et al. paper “Design Considerations for Generating Ammonia from Urea for NOx Control with SCRs”, AWMA 2007 Conference, showed that the water available for reaction when the reactor is held at a constant pressure decreases with increasing temperature.
Brooks et al. U.S. Pat. No. 6,761,868 describes a means to address the issue by controlling both temperature and pressure. In the instant invention reactor temperature is not controlled and pressure is adjusted as function of ammonia demand or temperature. The Brooks patent does show the pressure and temperature to maintain the concentration in the reactor at the feed concentration but does not show, as shown in this patent application, how the concentration in the reactor can be maintained at a desired value independent of the feed concentration.
Cooper et al. U.S. Pat. No. 6,077,491 discloses a process in which temperature and pressure are maintained by heat to generate a product gas but do not show how to maintain the liquid concentration in the reactor at near constant value.
Neither the Cooper nor the Brooks patents show the advantage of allowing the pressure in the reactor to vary as function of demand or as function of temperature.
A secondary issue is the additional heat needed to maintain higher urea concentration in solution. At 40% urea concentration the requirements for heat tracing of the urea feed system are reduced but reactor energy consumption is increased.
This invention identifies design considerations needed to be taken into account in order to maintain water balance in the reactor. This has the advantage that the product gas composition remains nearly constant during changes in gas production rate. Another of the advantages of the invention is the reactor can be operated such that for all demand conditions the product gas temperature is always above the gas dew point calculations for urea to ammonia generation processes. This result is in less corrosion and longer lasting reactors.